Tech UPTechnologyThe supermassive black hole in the Milky Way

The supermassive black hole in the Milky Way

In the center of the Milky Way lives a real monster that engulfs any star that dares to get too close to it and dominates the dynamics of the entire central region of the galaxy. This monster is a supermassive black hole with a mass of more than four million times that of our Sun (which already accounts for 99.8% of the mass of our solar system), located about 26,000 light years from the Earth . He goes by the name Sagittarius A*, but we haven’t always known as much about him.

The first clue that something impressive was prowling around that region of the Milky Way came in 1933 when Karl Janski , considered one of the fathers of radio astronomy , observed an intense source of radio waves coming from the constellation of Sagittarius , in the direction the one we thought was the galactic core. This source received the name of Sagittarius A. It was not until two decades later, in 1954 , that it was confirmed that this source must indeed be found in the very center of the galaxy and another two decades, in 1974 , that an object compact and bright in this region , which received the name of Sagittarius A* (read as Sagittarius A star, because in English the asterisk is called that).

Since the 1980s , it began to be suspected that this object must be a black hole (and a particularly large one) since it concentrated a mass millions of times greater than that of our Sun in a very small region . All observations of Sagittarius A* had to be made in radio, microwave or infrared waves , since the region in which it is located is not transparent to the visible part of the electromagnetic spectrum, that is, to “normal” light, the one that our eyes they are able to see. This is due to the large amount of dust between our solar system and the center of the Milky Way .

This dust is such as to cause an equivalent extinction of 25 magnitudes . So that you understand what this means, if a cloud of gas and dust with similar properties were placed between the Earth and the Sun, it would block the light of the star with such effectiveness that the Earth’s surface would receive the same light that it receives from only one of them. the brightest stars in the night sky . However, this same cloud that is completely opaque to visible light is transparent at longer wavelengths , and so we can observe it.

Since the 1990s , the orbits of some stars near the black hole began to be meticulously observed, studying the trajectory they traced over time. One of these stars, simply named S2 , aroused particular interest because it orbited exceptionally close to Sagittarius A*, taking only 16 years to complete one orbit. This allowed (and has allowed ever since) to place the mass of the black hole above four million times the mass of the Sun, confined in a space a few tens of times larger than the Earth’s orbit . These and related investigations led Reinhard Genzel and Andrea Ghez to share with Roger Penrose the 2020 Nobel Prize in Physics .

More recently, stars with defying orbits have been detected around Sagittarius A*, whose very existence puts Albert Einstein’s theory of general relativity to the test . The S2 star is the best studied of all the objects that occupy the area, but it is certainly not the only one, nor the most extreme. This star, which orbits the black hole at about 118 times the distance between the Sun and Earth (known as an astronomical unit or AU, about 150 million kilometers) or twice the maximum separation between Pluto and our star, reaches speeds of more than 2% of the speed of light .

However, the star S4714 would approach up to 12 astronomical units , approximately the same distance that separates Saturn from the Sun and could reach speeds close to 8% of the speed of light, or about 24,000 kilometers per second , at the most close to its orbit. At this speed, the star would cover the distance between the Earth and the Moon in about 16 seconds and the distance between the Earth and the Sun in less than two hours.

Sagittarius A* does not appear to be one of the so-called active galactic nuclei , which consist of supermassive black holes with extremely high levels of wave emission. In fact, these objects are the most luminous in the universe, discounting those that shine only for a short period of time, such as supernovae or gamma-ray bursts. The galaxy M87 , located about 53 million light-years from our own, does contain an active supermassive black hole at its center , which was photographed a few years ago in the famous first black hole photo.

References:

Genzel et al, 1994, The nucleus of our Galaxy, Reports on Progress in Physics, 57 (5), doi:10.1088/0034-4885/57/5/001

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